I. Field of the Invention
The current invention relates to wireless communication. More particularly, the present invention relates to a novel and improved method and apparatus for reducing call supervision time to allow faster reallocation of traffic channel resources in a wireless communication system.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the xe2x80x9cTIA/EIA-95A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, hereinafter referred to as the IS-95 standard. A system operating in accordance with the IS-95 standard is referred to herein as an IS-95 system. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein. Power control techniques in a CDMA multiple access communication system are disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d as well as in IS-95, and are well known in the art.
The term xe2x80x9cbase stationxe2x80x9d is used to refer to the hardware with which the subscriber stations communicate. The term xe2x80x9ccellxe2x80x9d refers to a geographic coverage area within which subscriber stations may communicate with a particular base station. Consequently, as a subscriber station moves from outside the coverage area of a base station towards the base station, the subscriber station eventually moves into the xe2x80x9cbase station""s cell.xe2x80x9d Each base station is typically located near the center of its cell. In a simple configuration, a base station transmits signals using a single carrier frequency to an entire cell. In order to increase call capacity, an additional base station may be installed at the same location to provide coverage within the same cell at a different carrier frequency. To increase capacity even further, a cell may be divided into radial regions much like pie slices. In this way, a cell may be xe2x80x9csectorizedxe2x80x9d, with each base station transmitting through directional antennas that cover only a portion of a cell. In the most common configuration, a cell is divided into three regions called sectors, with each sector covering a different 120-degree section of the cell. Each base station in a sectorized cell transmits at a single carrier within a single sector or within a single unsectorized cell.
In a CDMA system, a subscriber station communicates with a data network by transmitting data on the reverse link to a base station. The base station receives the data and can route the data to the data network. Data from the data network is transmitted on the forward link of the same base station to the subscriber station. The forward link refers to transmission from the base station to a subscriber station and the reverse link refers to transmission from the subscriber station to a base station. In IS-95 systems, separate frequencies are allocated for the forward link and the reverse link.
IS-95 systems utilize a plurality of different types of communication channels, including pilot, paging, and forward traffic channels. The availability of forward traffic channel resources determines how many different subscriber station calls can be supported by each base station. In order to maximize connection capacity, connection supervision techniques have been developed to free up traffic channel resources quickly and to prevent a subscriber station from acting as an in-band jammer should its traffic channel be lost unexpectedly. Such an unexpected call drop could result from movement of the subscriber station either out of coverage of a base station or through a tunnel that causes the loss of the traffic channel signal.
Traffic channel supervision in IS-95 includes two mechanisms, herein referred to as jammer prevention procedure and traffic channel recovery procedure. The jammer prevention procedure specifies the conditions under which a subscriber station must stop transmitting a reverse link signal. This procedure limits the length of time during which a subscriber station transmits a reverse link signal without being power-controlled by the base station. The traffic channel recovery procedure specifies the conditions under which a subscriber station will declare a loss of the traffic channel, ending the call. This second procedure allows the base station to reclaim and reuse a traffic channel when communication is suddenly lost to a subscriber station.
In IS-95, the jammer prevention procedure dictates that a subscriber station cease transmissions when it is not receiving a strong enough forward link signal to ensure good reverse link power control. If the subscriber station receives a specified number of consecutive erased frames (generally 12 frames), the subscriber turns off its transmitter. The transmitter may be turned back on after the subscriber station receives a specified number of good frames, such as 2 or3.
In IS-95 the traffic channel recovery procedure dictates that a subscriber station whose transmitter has been turned off in accordance with the jammer prevention procedure for a specified supervision time must declare its traffic channel lost. The supervision time for the traffic channel recovery procedure is typically around five seconds. Similarly, if the base station detects that a call with a subscriber station is no longer active, the base station will declare the traffic channel lost.
The above-described method allows recovery of traffic channel resources after a relatively short (five seconds) supervision time. One reason that this method works in an IS-95 system is that the base station continuously transmits new frames of information to each active subscriber station every 20 milliseconds, allowing the subscriber station to supervise on this continuous forward traffic stream. This approach is far less effective in a high data rate (HDR) system in which a base station transmits to a subscriber station only when the base station has data to send.
An exemplary HDR system for transmitting high rate digital data in a wireless communication system is disclosed in copending U.S. patent application Ser. No. 08/963,386, entitled xe2x80x9cMETHOD AND APPARATUS FOR HIGHER RATE PACKET DATA TRANSMISSIONxe2x80x9d (hereafter the ""386 application), assigned to the assignee of the present application and incorporated by reference herein. As described in the ""386 application, a base station transmits information to one subscriber station at a time, with the transmission rate depending on carrier-to-interference (C/I) measurements collected by the subscriber station. A subscriber station has only one connection with the base station, but that connection may comprise multiple traffic channels. The base station transmits information frames to a particular subscriber station only when the base station has data to send to that subscriber station. Thus, a subscriber station may maintain a connection with a base station on multiple traffic channels for a long period of time without receiving a frame of data from the base station.
In a system using such a transmit approach, a jammer prevention procedure could not rely on erasure rates, because the subscriber station cannot distinguish between receiving an erasure and not being sent a data frame. In addition, the supervision time necessary to reclaim traffic channel resources such a system would be less predictable and could far exceed five seconds. Methods of jammer prevention and of reducing supervision time in an HDR system are therefore highly desirable.
The present invention is directed to a novel and improved method and apparatus for high data rate wireless systems wherein data is transmitted according to the demands of a packet data network. The efficiency of the wireless system is improved by enabling quick reclamation and reuse of traffic channel resources when a subscriber station (herein referred to as an access terminal) shuts down or becomes suddenly inaccessible to network traffic.
In one aspect of the invention, in order to minimize reverse link jamming, each access terminal generates data rate control (DRC) values and monitors those generated DRC values. The DRC values vary according to carrier-to-interference (C/I) measurements made by the access terminal. When the C/I values measured at the access terminal fail to meet specified criteria, the access terminal generates a zero-rate DRC value indicating that the access terminal cannot decode forward link data at all. A DRC level of zero can also indicate that the access terminal is no longer within range of the base station, and is therefore no longer being effectively power controlled. When the DRC level remains at zero for a prolonged period, the access terminal turns off its transmitter to avoid becoming an uncontrolled in-band jammer. In an exemplary embodiment, the access terminal turns off its transmitter if the DRC level remains continuously at zero-rate for a xe2x80x9cTurn-Offxe2x80x9d period of approximately 240 milliseconds. The access terminal turns its transmitter back on after its DRC rate stays continuously above zero for a xe2x80x9cTurn-Onxe2x80x9d period, for example 13.33 or 26.67 milliseconds.
In another aspect of the invention, a wireless network communicates with an access terminal through a connection comprising one or more traffic channels. Each of the one or more traffic channels is allocated from a different base station belonging to the wireless network. The wireless network initiates the release of a connection with an access terminal by sending a release initiate message to the access terminal. The access terminal responds by sending a release message and then terminating its use of all traffic channels. In the event that the release initiate message or the release message is lost to communication error, the base station and access terminals use a traffic channel recovery procedure to limit the length of the supervision time. Minimizing the supervision time allows quick reclaiming and reuse of traffic channel resources by the base station.
In an exemplary embodiment, a wireless network controls supervision time by maintaining a minimum data frame transmission rate to each access terminal in the system. For example, if a maximum-zero-traffic period elapses without a data frame being sent to an access terminal, the wireless network transmits a null data frame to the subscriber station. If an access terminal does not successfully decode any data frame or null data frame on any of its traffic channels for a specified number maximum-zero-traffic periods, the access terminal declares a loss of its connection with the base station and stops transmitting. If the wireless system does not receive a release message after sending a release initiate message, it stops sending the data frames and null data frames to the access terminal. After a specified number of maximum-zero-traffic periods elapse, the wireless system reclaims the traffic channel resources allocated to the released access terminal.
In an advantageous embodiment, each base station of the wireless network instead controls supervision time by broadcasting a configuration packet to all active access terminals served by a base station. The configuration packet includes traffic channel allocation information indicating whether each of the base station""s traffic channels is allocated to an active access terminal. If an access terminal decodes a configuration packet indicating that one of its traffic channels has been deallocated, then the access terminal releases the traffic channel and optionally its connection with the wireless network. If the access terminal fails to successfully decode at least one configuration message for the duration of the supervision time, then the access terminal releases its traffic channels and its connection with the wireless network.